1. Field of the Invention
This invention relates to a device for changing an operating frequency of a microwave filter, said device being mounted in a wall of each cavity of a filter. The change in frequency can occur in response to temperature changes, thereby resulting in a temperature compensation microwave filter.
When a higher coefficient of thermal expansion is referred to in this specification, "higher" shall be interpreted to mean more positive (since coefficients of thermal expansion can be negative). Similarly, lower coefficients of thermal expansion means less positive. Similar terms have corresponding meanings.
2. Description of the Prior Art
It is known that temperature compensated filters can be compensated using irises made from bimetal materials (see Collins, et al., U.S. Pat. No. 4,488,132 issued Dec. 11, 1984; Atia, et al., U.S. Pat. No. 4,156,860 issued May 29, 1979 and Kick U.S. Pat. No. 4,677,403 issued Jun. 30, 1987). Temperature compensated filters that use bimetal end walls can be more complex to design than other temperature compensated filters. Further, in Japanese Patent No. 5-259719 (A) issued on Oct. 8, 1993, an adjustment screw made from dielectric material is provided with a hollow metallic thread. The dielectric body is fitted into the hollow thread. The dielectric screw penetrates into the cavity to compensate for changes in the cavity resonant frequency with temperature. The dielectric constant of the screw changes with temperature in such a fashion as to oppose changes in cavity resonant frequency that occur with temperature changes. The use of a dielectric screw can degrade the electrical performance of the filter.
A center frequency of a microwave filter changes as the operating temperature changes due to the expansion of materials with temperature. Filters are usually constructed of materials having a low coefficient of thermal expansion such as Invar (a trade mark). Invar is a relatively heavy material and when filters are used in satellite communication systems, the use of filters made from lighter materials is highly preferred. However, lighter materials, for example, aluminum, have a significantly higher coefficient of thermal expansion than Invar does. Therefore, lighter materials cannot reasonably be used for filters in satellite communication systems unless the change of center frequency can be reduced or eliminated by a temperature compensation device. While several temperature compensation devices are known, all of the previous devices have resulted in an increase in the insertion loss of the filter with which the device is used. When insertion loss of the filter increases, the transmitted power of the filter is reduced and the temperature of the filter is increased. Sometimes, it is desirable to change a center frequency of a filter without compensating for temperature changes.